Title: Design and resonator-assisted characterization of high performance lithium niobate waveguide crossings

Abstract: Waveguide crossings are elementary passive components for signal routing in photonic integrated circuits. Here, we design and characterize two multimode interferometer (MMI)-based waveguide crossings to serve the various routing directions in the anisotropic x-cut thin-film lithium niobate (TFLN) platform. To address the large measurement uncertainties in traditional cut-back characterization methods, we propose and demonstrate a resonator-assisted approach that dramatically reduces the uncertainty of insertion loss measurement(< 0.021 dB) and the lower bound of crosstalk measurement (-60 dB) using only two devices. Based on this approach, we demonstrate and verify TFLN waveguide crossings with insertion losses of < 0.070 dB and crosstalk of < -50 dB along all three routing directions at 1550 nm. The low-loss and low-crosstalk waveguide crossings in this work, together with the simple and efficient characterization strategy, could provide important layout design flexibility for future large-scale classical and quantum TFLN photonic circuits.